Gene therapy for neurodegenerative diseases has tremendous potential, but its success will rely on the development of vectors that are safe and that stably or transiently express enough levels of therapeutic transgene. Therefore, it is an extremely important object in the gene therapy for humans and animals to develop a system whereby a gene is transferred into target organs and target cells with a high efficiency. Methods for transferring a gene include the calcium phosphate method, DEAE-dextran method, cationic liposome method, electroporation method, etc., and especially methods for transferring a gene in vivo include a method using virus or liposome, or a direct transfer method. Among them, the gene transfer performed using “a virus vector” obtained by recombination of viral gene is extremely useful for the transfer of a gene into cells, for example, for gene therapy because of easy transfer procedure and its high transfer efficiency.
Virus vectors commonly used at present in gene therapy include retrovirus vector, herpes simplex virus (HSV) vector, adenovirus vector, and adeno-associated virus (AAV) vector, etc. In particular, along with the recent progress in analysis of brain functions using MRI and PET, there has been an increased demand for vectors capable of efficiently infecting non-dividing nerve cells and mediating a high level transgene expression in the infected cells. Therefore, adenoviral vector, herpes simplex viral vector, AAV, HIV, etc. have received considerable attention.
Although HSV has been reported to be capable of transferring a gene into ganglions in the peripheral nervous system, a problem remains on the amount of its expression (Gene Therapy, 1995, 2: 209-217). HIV infection of nerve cells has also been confirmed (Nature Biotechnology, 1997, 15: 871-875). Since the chromosomal position into which the HIV genome is inserted is hardly predictable, there are possibilities of damaging a normal gene, activating a cancer gene, and inducing excessive or suppressed expression of a desired gene.
AAV has been used for the brain treatment in Parkinson's disease (Exp. Neurol., 1997, 144: 147-156) and mucopolysaccharidosis type VII (ASGT meeting, 1998, Abstract No. 692). However, there have been reported an incomplete transfer of the introduced gene into the substantia nigra in Parkinson's disease and its insufficient expression in the brain in mucopolysccharidosis type VII.
Adenovirus has been most commonly used at present, and reported to be capable of transferring a gene into the pyramidal cell layer of hippocampus (Nature Medicine, 1997, 3: 997-1004). However, adenovirus has drawbacks, such as cytotoxicity and high immunogenicity.
On the other hand, since negative-sense RNA viruses, such as Sendai virus (hereinafter abbreviated as SeV), are not integrated into chromosomes, they do not activate cancer genes. Furthermore, since SeV is an RNA virus, it has advantages, such as protein expression in short time after infection and an extremely higher level expression of the transgene product compared with adenovirus.
GDNF is a neurotrophic factor that prevents dopaminergic neurons from dying and decreases dopamine-dependent behavioral deficits in rat and non-human primate models of Parkinson's disease (Bjorklund, A. et al. Neurobiol. Dis. 4: 186-200 (1997); Bowenkamp, K. E. et al., J. Comp. Neurol. 355: 479-489 (1995); Gash, D. M. et al., Nature 380: 252-255 (1996); Gash, D. M. et al. , Ann. Neurol. 44: S121-S125 (1998); Hebert, M. A. and Gerhardt, G. A. , J. Pharmacol. Exp. Ther. 282: 760-768 (1997)). However, in order for GDNF to be effective in a progressive neurodegenerative disease such as Parkinson's disease, chronically increased levels of GDNF near the dopaminergic neurons will most likely be needed. A recent study in Parkinson's patients infused intraventricularly with GDNF protein showed that GDNF protein did not penetrate the brain parenchyma to an appreciable extent (Kordower, J. H. et al., Ann. Neurol. 46: 419-424 (1999)). Gene delivery is the most promising approach for achieving chronically increased levels of GDNF near the dopamine neurons.